A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Immersion techniques have been introduced into lithographic systems to enable improved resolution of smaller features. In an immersion lithographic apparatus, a liquid layer of a liquid having a relatively high refractive index is interposed in a space between a projection system of the apparatus (through which the patterned beam is projected towards the substrate) and the substrate. The liquid covers at last the part of the wafer under the final lens element of the projection system. Thus, at least the portion of the substrate undergoing exposure is immersed in the liquid. The effect of the immersion liquid is to enable imaging of smaller features since the exposure radiation will have a shorter wavelength in the liquid than gas. (The effect of the liquid may also be regarded as increasing the effective numerical aperture (NA) of the system and also increasing the depth of focus.)
In commercial immersion lithography, the liquid is water. Typically the water is distilled water of high purity, such as Ultra-Pure Water (UPW) which is commonly used in semiconductor fabrication plants. In an immersion system, the UPW is often purified and it may undergo additional treatment steps before supply to the immersion space as immersion liquid. Other liquids with a high refractive index can be used besides water can be used as the immersion liquid, for example: a hydrocarbon, such as a fluorohydrocarbon; and/or an aqueous solution. Further, other fluids besides liquid have been envisaged for use in immersion lithography.
In this specification, reference will be made in the description to localized immersion in which the immersion liquid is confined, in use, to the space between the final lens element and a surface facing the final element. The facing surface is a surface of substrate or a surface of the supporting stage (or substrate table) that is co-planar with the substrate surface. (Please note that reference in the following text to surface of the substrate W also refers in addition or in the alternative to a surface of the substrate table, unless expressly stated otherwise; and vice versa). A fluid handling structure present between the projection system and the stage is used to confine the immersion to the immersion space. The space filled by liquid is smaller in plan than the top surface of the substrate and the space remains substantially stationary relative to the projection system while the substrate and substrate stage move underneath. Other immersion systems have been envisaged such as an unconfined immersion system (a so-called ‘All Wet’ immersion system) and a bath immersion system.
An alternative to immersion lithography is EUV lithography, in which the radiation beam is formed of EUV radiation, e.g. having a wavelength in the range of from 5 nm to 20 nm. EUV radiation can be generated by a plasma source or a free-electron laser, for example. In EUV lithography, the beam path, including the mask and substrate, are kept in a near-vacuum and reflective optical elements are mostly used. This is because EUV radiation is strongly absorbed by most materials. A low pressure of hydrogen gas may be present, e.g. to assist in cleaning contaminants when a plasma source is used.
In a lithographic apparatus, the production substrate, i.e. the substrate which is to be exposed, is usually held by a substrate holder which has a large number of small burls to support the substrate. The burls have a total area much less than the area of the substrate and serve two purposes. Firstly, because their total area is small relative to the area of the substrate any particulate contaminant that might fall on the substrate holder will most probably fall between burls and so will not distort the substrate unless it is larger than the height of the burls. Secondly, it is easier to ensure that the tops of the burls accurately conform to a flat plane than to ensure that a surface as large as the substrate is flat.
When the substrate is loaded onto and removed from the substrate holder, wear is caused to the burls. The rate of wear is unpredictable and is not uniform across the area of the substrate holder. Wear to the burls affects their height and therefore the flatness of the substrate supported by the substrate holder. Unflatness of the substrate during exposures can cause imaging errors such as increased overlay errors, but not in a predictable way.
If wear of the substrate holder is suspected, then wear can be detected by removing the substrate holder from the lithographic apparatus and using a confocal microscope or interferometry to map the surface contour of the substrate holder. This is a very time consuming procedure resulting in excessive downtime for the lithographic apparatus. It is also possible to perform a set of specific test exposures to determine if the substrate holder wear is causing overlay. However these require special reticles and themselves take up a significant amount of time, reducing throughput of the lithographic apparatus.